1. Field of the Invention
The present invention relates to an insulated gate field effect transistor, and in particular such a transistor having compensating strips in the area of the channel borders to decrease change in the channel width as a result of operating voltage fluctuations.
2. Description of the Prior Art
Conventional operation of insulated gate field effect transistors is such that a gate voltage supplied to the gate electrode creates a space charge zone in the channel region in the substrate forming an inversion layer in a region where the minority carriers dominate, the inversion layer being comprised of carriers of an opposite conductivity type to the substrate doping. In field effect transistors operating in the enhancement mode, the inversion layer becomes greater with increasing gate voltage so that current flow in the channel region increases.
Fluctuations in the substrate bias voltage supplied to the source result in a change in the potential of the source region which causes a change in the width of the inversion layer so that the effective width of the channel region of the transistor is also changed. A similar consequence occurs as a result of fluctuations in the gate and drain regions. An increasing difference between the potential of the source region and the reference potential results in a decrease of the effective channel width.
A field effect transistor operating in the depletion mode has a space charge zone formed without the presence of a gate voltage so that the changes in the effective width of the channel region as a result of fluctuations of operating potentials are even more pronounced.
A method for the manufacture of insulated gate field effect transistors is known from German OS No. 2,641,334 in which a semiconductor substrate is covered with a thick film insulating layer, portions of which are removed by photolithographic steps. A masking layer is applied over the entire semiconductor surface, including the openings in the insulating layer, and is subsequently removed from the thick film regions, so that the masking layer now covers only the openings. Ion implantation is then undertaken utilizing both the masking layer and the thick film insulation as a mask. After removal of the masking layer a thin film insulating layer is applied to the openings to form the insulated gate over the channel region.